An episode of ischemia followed by reperfusion such as occurs in traumatic injury, certain surgical procedures and various pathological conditions, results in injury in the local tissue and remote injury in the lungs as well. Three components of the innate immune system have been implicated in the injury, natural IgM, the complement system and mast cells (MC). These studies will initially focus on defining the role of the MC and its mediators in injury (both local and remote) following ischemia in the skeletal muscle and contrast that with the injury that occurs following ischemia in the intestine. The definition of the role of the MC and its mediators will be accomplished using animals lacking critical mediators due to targeted disruptions of the genes coding for 2 different secretory granule proteases, an enzyme critical for the production of heparin, the terminal enzymes in the pathway to synthesize prostaglandin D2 and leukotriene C4 and the cytokine tumor necrosis factor alpha. Confirmation of the role of the MC and the different mediators will be accomplished by engraftment of MC-deficient mice with the normal or mutant MC. The project will next define the interaction of the MC with the other innate immune system components in order to define the interaction, if any between these different pathways. These studies will make use of the knowledge and resources provided by the other parts of this program to define the aspects of interdependence and independence among the three components in producing the local and remote injury following ischemia in these two different tissues. Mice lacking various complement component and the natural antibody will provide the opportunity to define the interactions between these different pathways. Lastly, using the knowledge gained in these studies, the mechanism of protective of preconditioning will be explored in order to distinguish whether this short period of sichemia is protective due to desensitization, exhaustion of a critical mediator(s) or via some other mechanism. From these studies, a better understanding of the role of and interaction between the different innate immune system components will be achieved which will allow better interventions to prevent these injuries.